I personally have gotten a load of feedback from developers who aren't even planning to leave the 3.x series at all. Being that vB (after a stable release) is mostly supported by developers and owners alike, I think that there will be quite a nice balance going on there for quite some time.
Aside from the people with that mindset, there's highly modified websites who can't afford to or don't have the time or need for the major re-write, etc. Altogether, I believe the 3.x series will still have its players and it will still be game-on.
Just a matter of what else vB can do for us in 4.0 and whether it's worth the move